Power converters and in particular switched-mode power converters are used in a variety of applications to provide AC/DC and DC/DC conversion. For example, switched-mode power converters, also referred to as switched-mode power supplies (SMPS), are widely used in computer and mobile phone power supply units to provide the necessary operating voltages from typical 120V/240V AC mains lines.
Typical items of concern when designing power converters relate to conversion efficiency and cost. It should be readily apparent that power losses should be minimized to increase the overall efficiency of the converter and also to reduce the generation of heat, which may be difficult to dissipate depending on the design and the respective application.
It is known to operate switched-mode power converters in boundary conduction mode or short “boundary mode” (BCM). Unlike a continuous operation in CCM (continuous conduction mode), in boundary conduction mode it is aimed to operate the switch of the power converter when no or no substantial current flows through the switch. This operational mode reduces switching losses and also allows to use less expensive components, for example less expensive boost diodes in a boost switched-mode power converter setup due to no reverse-recovery losses. In addition, BCM also allows for power factor correction (PFC), in view that the input current follows the input voltage waveform.
A by-product of BCM is that the converter inherently uses a variable switching frequency. The frequency depends primarily on the selected output voltage, the instantaneous value of the input voltage, the parameters of the energy storage used, e.g., inductance or capacitance and the output power delivered to the load. The lowest frequency occurs at the peak of sinusoidal line voltage.
To operate properly in boundary conduction mode, it is necessary to determine the exact moment when the current through the switch reaches zero. In typical circuits, current sensing is used, for example using a current transformer, using a coupled inductance, such as a further winding close to a main inductance, or using CT or hall effect sensors. However, all of these approaches add cost and complexity to the circuit.